Integrated circuits are known which contain inverting/non-inverting voltage doubler charge pump circuitry for converting a unipolar supply voltage to a bipolar supply voltage of greater magnitude. Typically, the charge pump circuitry is fabricated on a single chip that receives the unipolar supply voltage (e.g. +5 v) and doubles it. The doubled voltage is then inverted, resulting in the bipolar voltage of increased magnitude (e.g. +/-10 v), which is used as a bipolar supply for on-chip circuitry, such as RS232 receivers and transmitters.
U.S. Pat. Nos. 4,679,134; 4,777,577; 4,797,899; 4,809,152; 4,897,774; 4,636,930 and 4,999,761 illustrate various implementations of such circuitry. The devices disclosed and claimed therein include switches which act during a first phase to direct a source voltage (unipolar) to a first capacitor. The first capacitor charges to the unipolar source voltage. During a second phase, switches act to place the source voltage in series with the voltage stored on the first capacitor to create a source of positive voltage which is approximately two times the unipolar source voltage. A reservoir capacitor is charged to the doubled positive voltage. The doubled positive voltage is stored on a transfer capacitor and subsequently transferred to a reservoir capacitor. Inversion of the doubled positive source voltage is then provided to generate the negative doubled portion of the bipolar voltage. Thus, in the referenced prior art, the doubled positive voltage is generated first and the inverted (i.e. negative) voltage is generated from the doubled positive voltage.
Use of the positive voltage to generate the negative voltage in charge pump circuits known in the prior art is disadvantageous with respect to the asymmetrical loading characteristic of the respective outputs. As illustrated in FIG. 1, in the prior art, given similar loading the negative voltage output is lower than the positive voltage output, i.e., the positive and negative voltage signals are asymmetrical with respect to their common reference. For instance, in the simulation of the prior art which is illustrated, a unipolar +5 V source is provided to a charge pump with a voltage doubler and inverter. The conventional voltage doubler and inverter doubles the source voltage to provide a doubled positive voltage and the negative voltage is generated by inverting the positive doubled voltage. While the prior art charge pump circuitry results in a doubled positive voltage C, averaging +9.15 volts under a 3 K ohm resistive load, the negative portion D generated by inverting the positive voltage averages -8.67 volts, under the same load.
The asymmetry of the doubled positive and inverted voltages negatively affects operational tolerance of circuitry constituting a load to the charge pump.